This project is concerned with the renal tubular transport of organic cations, some of which are drugs and some of which are neurohumors such as the biocation choline. The relation of the Tm for transport to competition between two cations has been studied and the results confirm to the theory proposed by Sperber. Tm's for TEA, triethylethanolamine and choline have been determined in vivo in the renal tubule. Bidirectional transport of choline has been detected and the enhancement of excretion of choline by other cations at low loads has been suggested to be caused by selective inhibition of the reabsorptive transport. Renal metabolism of choline has been studied and further quantitation will be made.